Introduction: The cellular basis of orthodontic tooth movement is complex, and is mediated by the biological responses of cells in the periodontal ligament (PDL) and alveolar bone. Appropriate homeostatic cytokine balance is essential for the safe and reliable induction of tooth movement. The endoplasmic reticulum (ER) plays a major role in maintaining homeostasis, with ER stress activating the unfolded protein response (UPR), potentially resulting in apoptotic cell death.

Objectives: 1) To validate a 3D-hydrogel model in which to culture human PDL cells and 2) To examine cell viability, apoptosis, and the expression of ER stress- and UPR-related genes following the application of mechanical strain (mimicking orthodontic tooth movement) to PDL cells.

Materials and Methods: Primary cultures of PDL cells were obtained from premolar teeth that were extracted from three individuals for orthodontic reasons. Viability and apoptosis assays were used to profile the time required by cultured PDL cells to establish themselves in hydrogel and assess their optimal seeding density. Non-strained PDL cells were used as controls. Optimal gel constitution and seeding density were determined and the cells were subjected to 24 hours of static mechanical strain (18% dimensional substrate elongation).

Results: A tendency for reduced cell viability was observed following the application of mechanical strain to both 2D and 3D cultures of PDL cells (cell viability of strained 2D and 3D cells was 83% and 73.1% respectively, of control values), while there was no difference in caspase activity. For monolayer samples, the gene LOX (involved in cross-linking of collagen and elastin) demonstrated a tendency to be upregulated following mechanical strain (mean fold-regulation = 9,22, p = 0.25). In 3D samples, a number of UPR-related genes were differentially upregulated; including CREB3L3 (mean fold-regulation = 1.91, p = 0.063), which plays a role in the acute inflammatory response, and DDIT3 (mean fold- regulation = 17.0, p = 0.438), a well-established pro-apoptotic factor in the UPR.

Conclusions: A model for the application of mechanical strain to 3D cultures of PDL cells has been validated. While a reduction in cell viability was observed following strain, an increase in caspase activity was not evident, thus the reduction in viability appears to be mediated via caspase-3/7-independent mechanisms. There is potential for the UPR to be involved in OTM, and future experiments could include increased strain periods and varying strain magnitudes.